1. Field of the Invention
The present invention relates to a method of laser peening a hidden surface of a workpiece, the hidden surface not being line-of-sight accessible to a laser beam for treatment, and, more particularly, to a method of laser peening dovetail slots in turbo machinery.
2. Description of the Related Art
In two components such as dovetail slots in turbo machinery, a significant cause of fatigue damage and failure in such components originates at the contact points therebetween. These contact points can be high stress, high fretting areas. As a result, fatigue cracks tend to initiate in such areas and can eventually propagate to an unacceptable damage threshold or failure.
Fretting fatigue is a process that occurs when two or more metallic components are in intimate contact with each other under active loading conditions. It occurs as a result of major axial load between two components accompanied by a cyclic load that introduces a small relative motion between the two components, such motion being on the order of microns. The fretting fatigue cracks tend to initiate in an area where the relative motion is greatest between the two surfaces. This area is typically at the edges of the contact area therebetween. Cracks generated from the cyclic load propagate in a direction at about 45° of the surface to the two components, due to the shear stress generated from the relative movement therebetween. The magnitude of this surface contact shear stress decreases with depth below the surface. Therefore, as the crack propagate into the component, the shear stress decreases until the cracks reach a depth at which the global stress begins to dominate.
When the global stress begins to dominate, the cracks begin to propagate perpendicular to the principal global tensile stress component. The crack can continue to propagate under the influence of the global tensile stress, possibly leading to eventual component failure.
Fretting fatigue is a relatively common problem in a number of components where there is a high contact stress between contact surfaces and vibratory stress that produces a slight relative motion between the two surfaces. Fretting fatigue often occurs at dovetail surfaces on mating turbine disk and blade attachment surfaces in aircraft jet engines. In the engines, centrifugal forces pull the blades against the surfaces of the dovetail slots of a turbine disk. This global loading is accompanied by a vibratory stress source that causes slight movement of the blade dovetail surfaces against the dovetail slot surfaces as the disk rotates. This movement can result in fretting and may lead to fretting crack initiation and, potentially, fatigue-induced failures of the disks and/or blades. As such, limiting or reducing the propagation of shear cracking caused by fretting fatigue can decrease maintenance costs and increase the life of a component.
One of the current measures taken to alleviate the fretting fatigue problem on dovetail slots has been the application of soft, compliant metallic coatings, such a copper-nickel-indium coatings, on the blade attachments. The soft, compliant coatings are applied as a thin layer to the blade attachment surface. Such coatings allow the peak contact stresses to be distributed uniformly across the contact area and reduce the relative motion between the surfaces. While these measures have helped extend service life, such coating have their own associated problems. These coatings generally degrade with each coating cycle and have to be reapplied to the attachment surfaces at periodic maintenance intervals.
Another technique that potentially increases fretting fatigue resistance is shot peening. However, two aspects limit shot peening as a method to increase fretting fatigue resistance in dovetail slots. The first limitation is that shot peening is only able to develop compressive residual stresses to relatively shallow depths. The compressive stress is limited to approximately 0.1 mm under typical processing conditions, where the shot media strike the surface at a near normal angle to the surface. However, the fretting fatigue cracks that are typically generated can be as deep as 0.1 mm and thus breach the residual compressive stress layer created by shock peening.
The second limitation associated with shot peening dovetail shock surfaces is the tight confines of the dovetail slot. The surfaces of the dovetail disk slots are difficult to access, thereby preventing a typical application of the shot peening process. In the tight confinement of the dovetail slot, the surfaces have to be shot peened at a lower incident angle, resulting in a lessened efficiency of the shot peening. Lowering the incident angle reduces the effective depth of the compressive stress layer created since the normal force (i.e., the force component perpendicular to the treated surface) of the shot media striking the surface is reduced significantly.
Laser peening processing of the fretting-prone areas on the dovetail slots of the disks is an excellent candidate technology to solve the fretting fatigue problem. Laser peening processing is capable of imparting deep residual compressive stresses (up to 1 mm deep) into the treated surface. The laser peening process can be applied at almost any angle to the surface as long as there is line-of-sight access to the surface to be processed. The surfaces can be processed with a low incident angle laser beam, if necessary, because the shockwave produced will propagate normal to the surface regardless of the laser beam incident angle.
Typical laser shock processing techniques and equipment can be found in U.S. Pat. No. 5,131,957 to Epstein along with that of U.S. Pat. No. 5,741,559 to Dulaney, each commonly assigned to the assignee of the present invention, the content of both of which is hereby incorporated by reference. U.S. Pat. No. 6,469,275 to Dulaney et al, which is commonly assigned along with the present invention and hereby incorporated by reference, discloses the use of laser peening to treat both the internal and external sections of a dovetail section (e.g., FIGS. 9 and 10 thereof).
Despite the significant potential offered by the laser peening processing to solve the fretting fatigue problem in dovetail slots, the immediate use of laser peening for treating the fretting surfaces has been inhibited because a portion of the dovetail surface has not been directly accessible with the laser beam.
What is needed is the art is the development of new, innovative beam delivery concepts that will promote delivery of laser peening energy to all surfaces, including those previously considered “hidden”, to permit producing compressive residual stresses into these surfaces via laser peening.